Re: Percentages
To quote Benford in ‘Naval Architecture for Non-Naval Architects’ 1991: “…the vessel’s turning center will seldom be found more than 20 percent of the vessel’s length from the bow, whereas the longitudinal center of gravity is usually somewhere near mid-length.”
You write that “The rule of thumb position of the pivot point when traveling at or around hull speed is one third of the distance along the keel at the waterline, measured back from the bow where it intersects the water. “ and “The phenomenon of the pivot point in displacement vessels is so well know and widely accepted (at least by professional mariners) that I suggest a search of the pertinent literature and some conversations with such persons, before looking for the forest between the trees, and before embarking on scientific research in what might not be such uncharted waters.”
I note that you have qualified your statement to read “when travelling at hull speed” which seems reasonable. If the pivot point moves from the CG forward once motion begins, surely it doesn’t instantly jump from 50% to 33%? If it winds up there at hull speed, so be it, although Benford disagrees on the location. One thing not mentioned in your article is the definition of the title: ‘Peripatetic Pivot Point’ – peripatetic means walking, or in terms of the pivot point, it moves around, so stating that it resides at a given position in general seems to be in error, but the hull speed qualification fixes that.
In the end, it seems the only thing widely accepted is that the effective turning center is forward of the CG – the assumed location obviously varies. I don’t have a problem with a rule of thumb (especially if it’s true), I’m just looking for a physical explanation. The several texts in Naval Arch that I’ve looked through don’t have one, but I will keep looking. The fact that it is well-accepted is fine, although I note that the concept of the ether was well-accepted in its day.
Per your suggestions, rest assured I am capable of calculating simple percentages. Also, as I am currently teaching the professional mariners of the future, I would like to be sure they are being taught physics, not anecdote. As far as my research plans, confirming and explaining (or denying) well-accepted bromides is one of the functions of experimental research, and a thorough literature search is the first step in any experiment. I don’t appreciate the implication that I am missing the forest for the trees – the fact that you can’t explain why the pivot point is where you say it is means there is work to be done.